calcMinFeeRecursive: return result at iteration limit when fee is reasonable

[Copilot]

When calcMinFeeRecursive hits the 50-iteration limit, it unconditionally returned FeeCalculationDidNotConverge. The result is still useful if the fee is valid — the error was too aggressive.

Definition of "reasonable fee": minFee <= txBodyFee <= 2 * minFee — the fee meets or exceeds the ledger-calculated minimum (so the transaction won't be rejected for insufficient fees) and does not exceed twice the minimum (ruling out a fee so high it likely signals a calculation error rather than ordinary oscillation).

Changes:

• go 0 base case — instead of discarding the final transaction state and returning an error, inspects it:
  • minFee <= txBodyFee <= 2 * minFee: validates outputs against min UTxO (consistent with Case 1) and returns Right tx
  • otherwise: returns Left FeeCalculationDidNotConverge
• Haddock — documents the full "reasonable fee" bounds at the iteration limit

The most likely scenario reaching go 0 is fee oscillation between two ledger-calculated minimum values (caused by a change output's CBOR size alternately shifting the tx byte count). In that case the two oscillating fee values differ by only a tiny fraction (a few bytes of encoding), so both bounds are easily satisfied and the result is useful to return.

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[Copilot]

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[carbolymer]

@copilot figure out an upper bound for the fee too!

[Copilot]

Done in 7a77450. The full "reasonable fee" bounds are now minFee <= txBodyFee <= 2 * minFee:

• Lower bound (>= minFee): ensures the ledger will accept the fee.
• Upper bound (<= 2 * minFee): rules out a fee so far above the minimum that it likely indicates a calculation error rather than ordinary oscillation.

In practice, at the iteration limit the fee was set to a prior ledger-computed minimum and will be within a tiny fraction of the current minimum (oscillation stems from a few bytes of CBOR encoding difference between iterations), so both bounds will be satisfied.